The present invention relates to a method of making a lithographic printing plate and more specifically to a method of developing treatment of a positive-working presensitized plate for use in making a lithographic printing plate (hereinafter referred to as "PS plate" which comprises a hydrophilized aluminum substrate provided thereon with an intermediate layer and a positive-working light-sensitive layer comprising an o-quinonediazide compound as well as a PS plate to be treated by the method.
Recently, an aqueous sodium silicate solution or an aqueous potassium silicate solution is utilized relatively advantageously as a developer composition for positive-working PS plate having a light-sensitive layer containing an o-quinonediazide compound as a light-sensitive material on the surface of an aluminum substrate. This is because that developing properties and stabilities can be controlled easily by varying the content of the silicon oxide and sodium oxide (or potassium oxide) which are the components of sodium silicate (or potassium silicate) and the concentrations thereof. However, when treatment using an automatic developing machine is repeated using such kinds of developer compositions, there are such problems as the formation of water-insoluble scum and sludge due to silicon oxide at the bottom of a developer tank and clogging of spray pipes and nozzles. Further, such developer compositions separate out as water-insoluble silica on or around the developer bath by drying, during stopping of the developing machine, or spattering of the solution which cause a variety of troubles. Further, when such developer compositions are exhausted, silicon oxide deposits out in the form of gel by neutralization, which leads to environmental pollution.
Developer compositions free of silicon oxide have been studied several times. However, there are such problems that when a developer composition free of silicon oxide is used, non-image portions of the resulting printing plate are contaminated during printing due to insufficient hydrophilicity of the non-image portions, or a substrate is easily etched with an alkali component in the developer composition due to the absence of silicon oxide on the substrate surface so that the aluminum component of the substrate separates out in a large quantity and thus the developing property is greatly damaged.
To eliminate the foregoing drawbacks, there have been adopted various means for hydrophilization such as a treatment with an alkali metal silicate as disclosed in U.S. Pat. No. 3,181,461; a treatment with fluorozirconic acid as disclosed in Japanese Examined Patent Publication (hereinafter referred to as "J. P. KOKOKU") No. Sho 36-22063 (=U.S. Pat. No. 2,946,683); and a treatment with a polyvinylsulfonic acid as disclosed in U.S. Pat. No 3,276,868. These methods can ensure sufficient hydrophilization of non-image portions of a lithographic printing plate and can eliminate the problem of etching of a substrate with an alkali, but cannot ensure sufficient adhesion between the hydrophilized substrate and a light-sensitive composition subsequently applied thereto and for this reason, the resulting lithographic printing plate can provide only several thousand copies.